In this study,zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone(YZSZ)in southern Xizang(Tibet)yielded an age of 247±3 Ma.According to whole rock geochemical and Sr-...In this study,zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone(YZSZ)in southern Xizang(Tibet)yielded an age of 247±3 Ma.According to whole rock geochemical and Sr-NdPb isotopic data,the Early Triassic samples could be divided into two groups:Group 1 with P-MORB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70253–0.70602,ε_(Nd)(t)values of 4.2–5.3,(^(206)Pb/^(204)Pb)_(t)ratios of 16.353–18.222,(^(207)Pb/^(204)Pb)_(t)ratios of 15.454–15.564,and(^(208)Pb/^(204)Pb)_(t)ratios of 35.665–38.136;Group 2 with OIB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70249–0.70513,ε_(Nd)(t)values of 4.4–4.9,(^(206)Pb/^(204)Pb)_(t)ratios of 17.140–18.328,(^(207)Pb/^(204)Pb)_(t)ratios of 15.491–15.575,and(^(208)Pb/^(204)Pb)_(t)ratios of 36.051–38.247.Group 2 rocks formed by partial melting of the mantle source enriched by a former plume,and assimilated continental crustal material during melt ascension.The formation of Group 1 rocks corresponds to the mixing of OIB melts,with the same components as Group 2 and N-MORBs.The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo-Tethys in southern Xizang(Tibet).展开更多
The Nianzha gold deposit, located in the central section of the Indus-Yarlung Tsangpo suture (IYS) zone in southern Tibet, is a large gold deposit (Au reserves of 25 tons with average grade of 3.08 g/t) controlled...The Nianzha gold deposit, located in the central section of the Indus-Yarlung Tsangpo suture (IYS) zone in southern Tibet, is a large gold deposit (Au reserves of 25 tons with average grade of 3.08 g/t) controlled by a E-W striking fault that developed during the main stage of Indo-Asian collision (-65-41 Ma). The main orebody is 1760 m long and 5.15 m thick, and occurs in a fracture zone bordered by Cretaceous diorite in the hanging wall to the north and the Renbu tectonic melange in the footwall to the south. High-grade mineralization occurs in a fracture zone between diorite and ultramafic rock in the Renbu tectonic melange. The wall-rock alteration is characterized by silicification in the fracture zone, serpentinization and the formation of talc and magnesite in the uitramafic unit, and chloritization and the formation of epidote and calcite in diorite. Quartz veins associated with Au mineralization can be divided into three stages. Fluid inclusion data indicate that the deposit formed from H20-NaCl-organic gas fluids that homogenize at temperatures of 203℃-347℃ and have salinities of 0.35wt%-17.17wt% NaCI equivalent. The quartz veins yield δ18Ofluid values of 0.15‰-10.45‰, low δDv-SMow values (-173%o to -96%o), and the δ13C values of-17.6‰ to -4.7‰, indicating the ore-forming fluids were a mix of metamorphic and sedimentary orogenic fluids with the addition of some meteoric and mantle-derived fluids. The pyrite within the diorite has δ34SV-CDT values of -2.9‰-1.9‰(average -1.1‰), 206pb/204pb values of 18.47- 18.64, 207pb/204pb values of 15.64-15.74, and 208pb/204pb values of 38.71-39.27, all of which are indicative of the derivation of S and other ore-forming elements from deep in the mantle. The presence of the Nianzha, Bangbu, and Mayum gold deposits within the IYS zone indicates that this area is highly prospective for large orogenic gold deposits. We identified three types of mineralization within the IYS, namely Bangbu-type accretionary, Mayum-type microcontinent, and Nianzha-type ophiolite-associated orogenic Au deposits. The three types formed at different depths in an aeeretionary orogenic tectonic setting. The Bangbu type was formed at the deepest level and the Nianzha type at the shallowest.展开更多
Cherts in the Zhongba melange of the western Yarlung Zangbo Suture Zone(YZSZ) contain well preserved radiolarian assemblages. These radiolarian assemblages indicate that the Zhongba melange has middle Jurassic-early C...Cherts in the Zhongba melange of the western Yarlung Zangbo Suture Zone(YZSZ) contain well preserved radiolarian assemblages. These radiolarian assemblages indicate that the Zhongba melange has middle Jurassic-early Cretaceous remnant, are coeval with those from the central and eastern parts of the YZSZ. Cherts from the Najiu area yield Aalenian to Aptian radiolarians, while cherts interbedded with siliceous mudstones from the Bielongjiala area yield Aptian radiolarians, indicating that terrigenousderived sediments were deposited during early Aptian. The above observations indicate that the entire YZSZ have a similar geochronological framework and thus they underwent similar geological evolution:(1) during the Jurassic, the Neo-Tethys was a wide ocean with pelagic sediments distal from continents;(2) during the Cretaceous(around 130-120 Ma), the Neo-Tethys started to subduct along the southern margin of the Lhasa block, and terrigenous-derived siliceous mudstone began deposition.展开更多
Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond hav...Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones(SSZ), where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.展开更多
The Cuobuzha high-Cr chromitites in the western segment of Yarlung Zangbo Suture Zone of Tibet are mainly hosted in the harzburgites as massive type, which are characterized by high concentrations of platinum group el...The Cuobuzha high-Cr chromitites in the western segment of Yarlung Zangbo Suture Zone of Tibet are mainly hosted in the harzburgites as massive type, which are characterized by high concentrations of platinum group elements(PGE) ranging from 380 to 577 ppb, and low Pd/Ir ratios(<0.1). In mid-ocean ridge basalts(MORB)-normalized spidergrams, chromites of the Cuobuzha chromitites are depleted in Al, Ga, V, Mg and Zn, and enriched in Mn and Cr, sharing similar patterns with those of ophiolitic boninites in the Bonin and Thetford Mines. Approximately 20 platinum group mineral(PGM) grains were discovered from the samples, including laurite, erlichmanite, Os-Fe alloy, cuproiridsite, and irarsite. The PGM assemblages indicate that sulfur fugacity was initially low enough to allow the precipitation of Os-Fe alloy and increased thereafter, with the fall in temperature. Primary Fe-Ni and Fe-Cr alloys, which are stable in a highly reduced environment, occur as inclusions within chromites or clinopyroxenes. Calculated results show that the parental magma has an intimate affinity with boninites. Based on our observations, a model is proposed wherein the Cuobuzha chromitites contain high-pressure and low-pressure chromites. Low-pressure chromites were formed via reaction between boninitic melts and peridotites, during which the high-pressure chromites hosting highly reduced minerals were mobilized by melts and were reallocated to podiform chromitites.展开更多
A discontinuous line of ophiolitic bodies occurs along the Yarlung Zangbo Suture Zone (YTSZ), which stretches across southern Tibet and beyond. This zone marks the locus of collision between Eurasia and India in the E...A discontinuous line of ophiolitic bodies occurs along the Yarlung Zangbo Suture Zone (YTSZ), which stretches across southern Tibet and beyond. This zone marks the locus of collision between Eurasia and India in the Early Cenozoic. Should we assume that the entire Tethyan Ocean basin that lay between these two continental blocks was oceanic or might it have been more complex? Fragments of any terranes that developed within this once extensive ocean potentially lie within the YTSZ. Detailed investigations over the past three field seasons reveal the presence of several terranes distributed along this zone. Work is currently underway to analyze the nature of individual terranes and the timing of any inter\|relationships.展开更多
The Yarlung Zangbo Suture Zone(YZSZ)in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the nort...The Yarlung Zangbo Suture Zone(YZSZ)in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the north(Dupuis et al.,2005;Yang et al.,2011).In the western part of the YZSZ,the Northern and the Southern sub-belts form two sub-parallel zones of mafic–ultramaficrockassemblageswithoverlapping crystallization ages(Xiong et al.,2011;Hebért et al.,2012;Liu et al.,2015).The upper mantle section of the Cuobuzha ophiolite in the northern sub-belt of the Yarlung–Zangbo Suture Zone(YZSZ)in SW Tibet comprises mainly clinopyroxene(cpx)–rich and depleted harzburgites.Spinels in the cpx-harzburgites show lower Cr#values(12.6–15.1)than the spinels in the harzburgites(26.1–34.5),and the cpx-harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites.The harzburgites have subchondritic Os isotopic compositions(0.11624–0.11699),whereas the cpx-harzburgites have suprachondritic 187Os/188Os ratios(0.12831–0.13125)with higher Re concentrations(0.380-0.575 ppb).The cpx-harzburgites plot in a Re vs.Al2O3 diagram as a result of subsequent addition of Re following the last partial melting event that occurred during mid-ocean ridge melt evolution processes(Uysal et al.,2015).Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-ocean ridge type upper mantle units,their melt evolution trends reflect different mantle processes.The cpx-harzburgites formed from low-degree partial melting(;%)of a primitive mantle source,and they weresubsequently modified by melt–rock interactions in a mid-ocean ridge environment.The depleted harzburgites,on the other hand,were produced by re-melting of the cpx-harzburgites,which later interacted with MORB-or island arc tholeiite(IAT)-like melts(Fig.1)possibly in a trench-distal backarc spreading center.Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neotethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.展开更多
The Ceno\|Tethys is a southern branch of the Tethyan ocean and existed in the Mesozoic and early Cenozoic times. The evolution of the Ceno\|Tethys has been discussed based mainly on geological and paleontological data...The Ceno\|Tethys is a southern branch of the Tethyan ocean and existed in the Mesozoic and early Cenozoic times. The evolution of the Ceno\|Tethys has been discussed based mainly on geological and paleontological data obtained from shallow marine sediments on the both sides of the ocean. Pelagic sediments deposited in a deep ocean basin of the Ceno\|Tethys are often incorporated in suture zones. However, geological and paleontological data from the pelagic sediments, which enable us to elucidate the paleoceanography and tectonic evolution of the Ceno\|Tethys, are still limited. The Xialu chert crops out about 30 km south of Xigaze and occupies the southern marginal part of the E—W trending Yarlung Zangbo Suture zone. As a result of radiolarian biostratigraphic research along two continuous sections, we identified seven different aged radiolarian assemblages from pelagic and hemipelagic sediments ranging early Middle Jurassic (Aalenian) to Early Cretaceous. The Aalenian radiolarian fauna is the oldest known record so far for the Xialu chert. It is noteworthy to point out that the fauna came from a chert sample which contains no terrigenous elements other than clay minerals. There is a possibility that radiolarian faunas older than Aalenian age will be discovered from the Xialu chert in the future.展开更多
Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Creta...Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous.Older ophiolite or ophiolitic mélange(e.g.Triassic)to reveal the initial evolution the Neo-Tethyan ocean within the YZSZ have rarely been documented so far.The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic subbelt,the Zhongba terrane and the southern ophiolitic sub-belt.In this study,we document structural,petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub-belt of the western YZSZ.Two lithos-tectonic sub-units,the southern ophiolitic complex and the northern ophiolitic mélange,are recognized in the Mayoumu massif in terms of structural pattern and petrology.LA-ICP-MS zircon U-Pb dating of gabbro from the ophiolitic mélange yields an age of 243±1 Ma with zirconεHf(t)values of+7.9 to+13.2.Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zirconεHf(t)values ranging from+10.8 to+15.0 and+12.3 to+15.4,respectively.Geochemically,rare earth elements(REEs)patterns show that these mafic intrusions are similar to those of the normal mid-ocean-ridge basalt(N-MORB).Enrichment of fluidmobile elements(e.g.Rb and Ba)and depletion of Nb,and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab.Our data strongly confirms that the Neo-Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least.The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo-Tethyan ocean could be as a continuing process during 120–130 Ma.Emplacement and preservation of older ophiolitic recorder during evolution of the Neo-Tethyan Ocean may be closely related to the occurrence of the Zhongba micro-terrane within the western YZSZ.展开更多
The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is...The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(>20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.展开更多
Meso\| and microstructural characteristics of cataclastic fault rocks developed in the Xialu chert are described. The Xialu chert represents pelagic and deep marine sediments (Middle Jurassic—Lower Cretaceous), and o...Meso\| and microstructural characteristics of cataclastic fault rocks developed in the Xialu chert are described. The Xialu chert represents pelagic and deep marine sediments (Middle Jurassic—Lower Cretaceous), and occupies the southern marginal part of the E—W trending Yarlung Zangbo suture zone. The apparent total thickness of siliceous deposits exceeds 1km. The current study is concentrated along the two measured sections, Xialu\|E and Xialu\|W. The Xialu\|E section is composed mainly of red chert associated with reddish purple siliceous mudstone and greenish gray mudstone. The beds steeply dip north or south. The 45m section studied is divided into three units (chert unit, chert\|siliceous mudstone unit, and melange unit from north to south). They are in fault contact with each other. In the southern half of the chert unit and the northernmost of the chert\|siliceous mudstone unit, a 15m thick cohesive cataclasite zone can be defined. The cataclasites are composed of chert fragments in a fine grained matrix, and deformed with jigsaw puzzle structures. The chert\|siliceous mudstone unit is made up of four slabs of chert\|siliceous mudstone sequence. Mesoscopic duplex zones in thickness from 1 to 3m are recognized at slab boundaries. Flat and ramp structure and associated P foliations, R1 shears, Y surfaces are developed. Shear spacing range from 1 to 10cm. The slip senses are dextral strike\|slip movement along these duplex zones. The cataclasite zone is cut by the duplex zone along the southern boundary.展开更多
The Yarlung Zangbo suture zone(YZSZ)separates Indian plate and its northern passive margin units to the south from Eurasian plate and its active continental margin units of Xigaze forearc basin and Gangdese batholith ...The Yarlung Zangbo suture zone(YZSZ)separates Indian plate and its northern passive margin units to the south from Eurasian plate and its active continental margin units of Xigaze forearc basin and Gangdese batholith to the north(Xu et al.,2015;Yang et al.,2015).The western YZSZ in southern Tibet is divided by the Zhongba terrane into the northern belt(NB)and southern belt(SB).Ophiolites in the NB are dismembered as ophiolitic mélanges.Peridotite,cumulated gabbro,ocean plate stratigraphy(OPS)of seamount remnants and pelagichemipelagic sequence as blocks in serpentinite matrix are mainly observed,from west to east,in Dajiweng,Baer,Kazhan,Cuobuzha,Zhalai,Gongzhu.Ophiolites in the SB are absent ophiolitic units of sheeted dikes and MORB-like pillow lavas,occur as much larger peridotite massifs(i.e.,Dongbo,400 km^2;Purang,650 km^2;Xiugugabu,700 km^2;Dangqiong,300 km^2)which are intruded by mafic dike swarms and overlain by volcanic sedimentary OPS(Liu et al.,2018).We propose that the SB mafic–ultramafic rocks and volcanic sedimentary OPS represent fragments of an early Cretaceous continental margin ophiolite whose magmatic evolution was influenced by 140–137 Ma plume magmatism(Liu et al.,2015;Zheng et al.,2019).Relics of Late Paleocene to very Early Eocene deep-marine basin were developed in Saga and Gyirong(Ding,2003;Li et al.,2018).In contract,the NB ophiolitic mélanges report a travel log of an oceanic plate ranging from Middle Triassic to Early Cretaceous.展开更多
The Dongbo ophiolite in the western part of the Yarlung-Zangbo suture zone in southern Tibet rests tectonically on the middle-late Triassic and Cretaceous flysch units,and consist mainly of peridotites,mafic dikes,
The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rock...The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine (Fos9.7-91.2), orthopyroxene (Enss-92), clinopyroxene (En4-49Wo47-slFS2-4) and spinel [Mg^#=lOOxMg/(Mg+Fe)]=49.1-70.7; Cr^#=(100xCr/(Cr+Al)=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content (0.08-0.62 ppm). Their "spoon-shaped" primitivemantle normalized REE patterns with (La/Sm)N being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites (18.19-50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with 187Os/18Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge (MOR) and were later modified in supra-subduction zone (SSZ) environment.展开更多
The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic.However,the evolution of the Neo-Tethys is not well constrained,in...The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic.However,the evolution of the Neo-Tethys is not well constrained,in particular whether its opening occurred in the Permian or the Triassic and whether a plume was involved with its opening or not.In this study,we present geochronological and geochemical data for mafic igneous rocks in mélanges along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet to constrain the timing and mechanism of opening the Neo-Tethys Ocean.Based on field observations,the YZSZ mélanges can be divided into three segments.The western(west of Zhongba)and eastern(Sangsang-Renbu)segments are composed of ocean plate stratigraphy representing accretionary complexes that formed during subduction of Neo-Tethyan oceanic lithosphere beneath the southern margin of the Asian continent.Mélanges in the central segment(Zhongba-eastern Saga)typically have a siliciclastic matrix,and represent Tethyan Himalayan strata that were structurally mixed with the southern margin of the Asian continent.Based on our and previously published geochemical data,the mafic rocks in the YZSZ mélanges are ocean island basalt(OIB)-like,with ages in the Late Permian-Middle Triassic,the Middle-Late Jurassic,and the Early Cretaceous,respectively.An OIB-like block with an age of ca.253 Ma is identified from the Zhongba mélanges in the western segment,and it is the oldest OIB lithology yet identified in the YZSZ mélanges related to the evolution of the Neo-Tethys Ocean.Geochemical features indicate that this OIB-like block is distinct from typical OIBs and would be formed during continental rifting to incipient seafloor spreading.In the framework of plate divergent-convergent coupling systems and based on literature data for early Middle Triassic seamounts,radiolarian cherts,and normal mid-ocean ridge basalt-like oceanic crust,we conclude that opening of the Yarlung Zangbo Neo-Tethys Ocean would mainly occur at~250–243 Ma in the Early Triassic,not later than the early phase of Middle Triassic.In addition,a mantle plume was not involved in opening the Yarlung Zangbo Neo-Tethys Ocean.On the other hand,we have also identified a suite of ca.160 Ma OIB-like basaltic sills from the Bainang mélanges in the eastern segment,which is the same age as the OIB lithologies previously reported in the Zhongba mélanges.Based on the sill-like occurrence and absence of plume-related rock associations in this region,the Bainang OIB-like rocks might result from Middle-Late Jurassic continental rifting in northern Gondwana.Magmatism related to this tectonic event is preserved in both the YZSZ mélanges and Himalayan strata,but its tectonic significance requires further investigation.Based on this study of the YZSZ mélanges and the previous studies of YZSZ ophiolites,Gangdese belt igneous rocks,and sedimentary rocks,we have reconstructed the entire Wilson Cycle of the Yarlung Zangbo Neo-Tethys Ocean,mainly involving continental rifting and ocean opening,subduction initiation,ultraslow-spreading ridge-trench conversion,subduction re-initiation,and oceanic closure and initial India-Asia collision for the tectonic emplacement of ophiolites.These processes were associated not only with magmatic flare-ups and lulls in the Gangdese belt but also with two stages of ophiolite obduction.Our data therefore provide new insights into the evolution of the Yarlung Zangbo Neo-Tethys Ocean and related Tethyan geodynamics.展开更多
Three radiolarian fauna, aged as the Middle-Late Triassic, Late Jurassic-Early Creta- ceous and Cretaceous radiolarian fauna, have been recognised in the radiolarian cherts from the middle sector of the Yarlung Zangbo...Three radiolarian fauna, aged as the Middle-Late Triassic, Late Jurassic-Early Creta- ceous and Cretaceous radiolarian fauna, have been recognised in the radiolarian cherts from the middle sector of the Yarlung Zangbo suture zone, southern Tibet, China. The average contents of SiO2 in the radiolarian cherts of the Middle-Late Triassic and Late Jurassic-Early Cretaceous are 90.24% and 92.58% respectively, with average ratios of Al/(Al+Fe+Mn) as 0.75 and 0.74 respectively, the average ratios of MnO/TiO2 as 0.36 and 1.24, the average ratios of Ce/Ce* as 1.15 and 1.03, and the average ratios of LaN/CeN as 0.85 and 0.93. These geochemical features indicate that both of them are biogenic, deposited in a continental margin basin. The SiO2 content of the Early Cretaceous radiolarian chert is 94.12%, with the ratio of Al/(Al+Fe+Mn) as 0.59, ratio of MnO/TiO2 as 4.30, ratio of Ce/Ce* as 0.60, ratio of LaN/CeN as 1.59, which imply that the chert is biogenic and was deposited in a pelagic basin. The Middle-Late Triassic association of the radiolarian chert and turbidites as well as their geochemical characteristics indicates the existence of a strong rifting marginal basin in the belt of the Yarlung Zangbo River then. The association of radiolarian chert and bedded basalt indicate an initial Tethyan ocean basin in southern Tibet during the Late Jurassic-Early Cretaceous. The early Cretaceous radiolarian chert coexisting with pillow basalt in top of the ophiolite suite represents sediments from the oceanic Tethyan basin.展开更多
Trench basin,as an important sedimentary repository in oceanic subduction zones,documents faithfully the evolution of paleodrainage and paleogeographic information.Because of the frequent intense deformation during an...Trench basin,as an important sedimentary repository in oceanic subduction zones,documents faithfully the evolution of paleodrainage and paleogeographic information.Because of the frequent intense deformation during and after deposition,the recognition of trench-basin strata in orogenic belts is quite challenging.Several trench-fill deposits have been identified from the Yarlung Zangbo suture in southern Tibet,which can be classified into two types based on major differences in formation timing and tectonic setting.The first type developed during subduction of the Neotethyan oceanic slab in the Cretaceous(e.g.,the Jiachala,Rongmawa,and Luogangcuo formations),and the second type developed during the initial stage of the India-Asia collision in the Palaeogene(e.g.,the Sangdanlin-Zheya formations).The former was originally deposited on the subducting oceanic crust and then accreted as tectonic slices into the subduction complex;the latter was deposited unconformably on the continental margin of the subducting Indian plate and then involved in the subduction complex during the continental collision.Typical lithologies of trench-basin fills include abyssal chert,siliceous shale,silty to sandy turbidites,debris flows deposits,and slump deposits without carbonate.Detritus feeding these basins were chiefly from the uplifted terrane in the upper plate.This paper summarizes the geological features of trench basins developed in southern Tibet and proposes criteria for recognizing trench-basins in collisional orogens.展开更多
Understanding the nature of parental melts for pyroxenite veins in supra-subduction zone(SSZ)ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones.The Zedang ophiolitic m...Understanding the nature of parental melts for pyroxenite veins in supra-subduction zone(SSZ)ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones.The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro,diabase,and basalt.Veins of two pyroxenite varieties cut the southern part of the Zedang massif.These pyroxenite rocks have different geochemical characteristics,where the first variety(type-I)has relatively higher contents of SiO_(2)(51.82–53.08 wt%),MgO(20.08–23.23 wt%),andΣPGE(3.42–13.97 ppb),and lower Al_(2)O_(3)(1.59–2.28 wt%)andΣREE(1.63–2.94 ppm).The second pyroxenite variety(type-II)is characterized by SiO_(2)(45.44–49.61 wt%),Mg O(16.68–19.78 wt%),Al_(2)O_(3)(4.24–8.77 wt%),ΣPGE(14.46–322.06 ppb),andΣREE(5.82–7.44 ppm).Pyroxenite type-I shows N-MORB-like chondritenormalized REE patterns.Zircon U-Pb ages of pyroxenite type-I(194±10 Ma),associated ophiolitic gabbro(135.3±2.0 Ma),and plagiogranite(124.2±2.3 Ma)evidently imply episodic evolution of the Zedang ophiolites.The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction-initiated hydrous melting of metasomatized sub-arc mantle,later overprinted by sub-slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times.The geochemical variations in pyroxenite vein composition,coupled with age differences amongst the other ophiolite units,may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.展开更多
基金the National Natural Science Foundation of China(Grant Nos.91955206,41603038)Second Tibetan Plateau Scientific Expedition and Research program(Grant No.2019QZKK0803)+2 种基金Scientific Research Foundation for Advanced ScholarsWest Yunnan University of Applied Sciences(Grant No.2022RCKY0004)Yunnan Fundamental Research Projects(Grant No.202301AT070012).
文摘In this study,zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone(YZSZ)in southern Xizang(Tibet)yielded an age of 247±3 Ma.According to whole rock geochemical and Sr-NdPb isotopic data,the Early Triassic samples could be divided into two groups:Group 1 with P-MORB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70253–0.70602,ε_(Nd)(t)values of 4.2–5.3,(^(206)Pb/^(204)Pb)_(t)ratios of 16.353–18.222,(^(207)Pb/^(204)Pb)_(t)ratios of 15.454–15.564,and(^(208)Pb/^(204)Pb)_(t)ratios of 35.665–38.136;Group 2 with OIB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70249–0.70513,ε_(Nd)(t)values of 4.4–4.9,(^(206)Pb/^(204)Pb)_(t)ratios of 17.140–18.328,(^(207)Pb/^(204)Pb)_(t)ratios of 15.491–15.575,and(^(208)Pb/^(204)Pb)_(t)ratios of 36.051–38.247.Group 2 rocks formed by partial melting of the mantle source enriched by a former plume,and assimilated continental crustal material during melt ascension.The formation of Group 1 rocks corresponds to the mixing of OIB melts,with the same components as Group 2 and N-MORBs.The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo-Tethys in southern Xizang(Tibet).
基金provided by the National Key Research and Development Program of China "Deep Structure and Ore-forming Process of Main Mineralization System in Tibetan Orogen"(2016YFC0600300)the National Basic Research Program of China (2011CB403104)+1 种基金the China Geological Survey (12120113037901)the National Natural Science Foundation of China(41320104004) and(41503040)
文摘The Nianzha gold deposit, located in the central section of the Indus-Yarlung Tsangpo suture (IYS) zone in southern Tibet, is a large gold deposit (Au reserves of 25 tons with average grade of 3.08 g/t) controlled by a E-W striking fault that developed during the main stage of Indo-Asian collision (-65-41 Ma). The main orebody is 1760 m long and 5.15 m thick, and occurs in a fracture zone bordered by Cretaceous diorite in the hanging wall to the north and the Renbu tectonic melange in the footwall to the south. High-grade mineralization occurs in a fracture zone between diorite and ultramafic rock in the Renbu tectonic melange. The wall-rock alteration is characterized by silicification in the fracture zone, serpentinization and the formation of talc and magnesite in the uitramafic unit, and chloritization and the formation of epidote and calcite in diorite. Quartz veins associated with Au mineralization can be divided into three stages. Fluid inclusion data indicate that the deposit formed from H20-NaCl-organic gas fluids that homogenize at temperatures of 203℃-347℃ and have salinities of 0.35wt%-17.17wt% NaCI equivalent. The quartz veins yield δ18Ofluid values of 0.15‰-10.45‰, low δDv-SMow values (-173%o to -96%o), and the δ13C values of-17.6‰ to -4.7‰, indicating the ore-forming fluids were a mix of metamorphic and sedimentary orogenic fluids with the addition of some meteoric and mantle-derived fluids. The pyrite within the diorite has δ34SV-CDT values of -2.9‰-1.9‰(average -1.1‰), 206pb/204pb values of 18.47- 18.64, 207pb/204pb values of 15.64-15.74, and 208pb/204pb values of 38.71-39.27, all of which are indicative of the derivation of S and other ore-forming elements from deep in the mantle. The presence of the Nianzha, Bangbu, and Mayum gold deposits within the IYS zone indicates that this area is highly prospective for large orogenic gold deposits. We identified three types of mineralization within the IYS, namely Bangbu-type accretionary, Mayum-type microcontinent, and Nianzha-type ophiolite-associated orogenic Au deposits. The three types formed at different depths in an aeeretionary orogenic tectonic setting. The Bangbu type was formed at the deepest level and the Nianzha type at the shallowest.
基金supported by the Research Center for Tibetan Plateau Geology, China Geological Survey(201401)Funds for the Central Universities (Grant No. 292016004)China Geological Survey(Grant Nos. 12112011086037 and 1212011221072)
文摘Cherts in the Zhongba melange of the western Yarlung Zangbo Suture Zone(YZSZ) contain well preserved radiolarian assemblages. These radiolarian assemblages indicate that the Zhongba melange has middle Jurassic-early Cretaceous remnant, are coeval with those from the central and eastern parts of the YZSZ. Cherts from the Najiu area yield Aalenian to Aptian radiolarians, while cherts interbedded with siliceous mudstones from the Bielongjiala area yield Aptian radiolarians, indicating that terrigenousderived sediments were deposited during early Aptian. The above observations indicate that the entire YZSZ have a similar geochronological framework and thus they underwent similar geological evolution:(1) during the Jurassic, the Neo-Tethys was a wide ocean with pelagic sediments distal from continents;(2) during the Cretaceous(around 130-120 Ma), the Neo-Tethys started to subduct along the southern margin of the Lhasa block, and terrigenous-derived siliceous mudstone began deposition.
基金funded by grants from Sinoprobe-05-02 from the Ministry of Science and Technology of China,the NSF China(Nos.41502062,40930313,40921001,41202036)the China Geological Survey(Nos.12120115027201,12120114061801,2014DFR2127C)from the Institute of Geology,Chinese Academy of Geological Sciences(J1526)
文摘Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones(SSZ), where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.
基金funded by grants from the Ministry of Science and Technology of China (2014DFR21270)from the China Geological Survey (DD20160023-01 and DD20160022-01)+1 种基金from the National Natural Science Foundation of China (41802034)from the National Science Foundation of China (41720104009, 41672063, 41773029).
文摘The Cuobuzha high-Cr chromitites in the western segment of Yarlung Zangbo Suture Zone of Tibet are mainly hosted in the harzburgites as massive type, which are characterized by high concentrations of platinum group elements(PGE) ranging from 380 to 577 ppb, and low Pd/Ir ratios(<0.1). In mid-ocean ridge basalts(MORB)-normalized spidergrams, chromites of the Cuobuzha chromitites are depleted in Al, Ga, V, Mg and Zn, and enriched in Mn and Cr, sharing similar patterns with those of ophiolitic boninites in the Bonin and Thetford Mines. Approximately 20 platinum group mineral(PGM) grains were discovered from the samples, including laurite, erlichmanite, Os-Fe alloy, cuproiridsite, and irarsite. The PGM assemblages indicate that sulfur fugacity was initially low enough to allow the precipitation of Os-Fe alloy and increased thereafter, with the fall in temperature. Primary Fe-Ni and Fe-Cr alloys, which are stable in a highly reduced environment, occur as inclusions within chromites or clinopyroxenes. Calculated results show that the parental magma has an intimate affinity with boninites. Based on our observations, a model is proposed wherein the Cuobuzha chromitites contain high-pressure and low-pressure chromites. Low-pressure chromites were formed via reaction between boninitic melts and peridotites, during which the high-pressure chromites hosting highly reduced minerals were mobilized by melts and were reallocated to podiform chromitites.
文摘A discontinuous line of ophiolitic bodies occurs along the Yarlung Zangbo Suture Zone (YTSZ), which stretches across southern Tibet and beyond. This zone marks the locus of collision between Eurasia and India in the Early Cenozoic. Should we assume that the entire Tethyan Ocean basin that lay between these two continental blocks was oceanic or might it have been more complex? Fragments of any terranes that developed within this once extensive ocean potentially lie within the YTSZ. Detailed investigations over the past three field seasons reveal the presence of several terranes distributed along this zone. Work is currently underway to analyze the nature of individual terranes and the timing of any inter\|relationships.
文摘The Yarlung Zangbo Suture Zone(YZSZ)in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the north(Dupuis et al.,2005;Yang et al.,2011).In the western part of the YZSZ,the Northern and the Southern sub-belts form two sub-parallel zones of mafic–ultramaficrockassemblageswithoverlapping crystallization ages(Xiong et al.,2011;Hebért et al.,2012;Liu et al.,2015).The upper mantle section of the Cuobuzha ophiolite in the northern sub-belt of the Yarlung–Zangbo Suture Zone(YZSZ)in SW Tibet comprises mainly clinopyroxene(cpx)–rich and depleted harzburgites.Spinels in the cpx-harzburgites show lower Cr#values(12.6–15.1)than the spinels in the harzburgites(26.1–34.5),and the cpx-harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites.The harzburgites have subchondritic Os isotopic compositions(0.11624–0.11699),whereas the cpx-harzburgites have suprachondritic 187Os/188Os ratios(0.12831–0.13125)with higher Re concentrations(0.380-0.575 ppb).The cpx-harzburgites plot in a Re vs.Al2O3 diagram as a result of subsequent addition of Re following the last partial melting event that occurred during mid-ocean ridge melt evolution processes(Uysal et al.,2015).Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-ocean ridge type upper mantle units,their melt evolution trends reflect different mantle processes.The cpx-harzburgites formed from low-degree partial melting(;%)of a primitive mantle source,and they weresubsequently modified by melt–rock interactions in a mid-ocean ridge environment.The depleted harzburgites,on the other hand,were produced by re-melting of the cpx-harzburgites,which later interacted with MORB-or island arc tholeiite(IAT)-like melts(Fig.1)possibly in a trench-distal backarc spreading center.Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neotethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.
文摘The Ceno\|Tethys is a southern branch of the Tethyan ocean and existed in the Mesozoic and early Cenozoic times. The evolution of the Ceno\|Tethys has been discussed based mainly on geological and paleontological data obtained from shallow marine sediments on the both sides of the ocean. Pelagic sediments deposited in a deep ocean basin of the Ceno\|Tethys are often incorporated in suture zones. However, geological and paleontological data from the pelagic sediments, which enable us to elucidate the paleoceanography and tectonic evolution of the Ceno\|Tethys, are still limited. The Xialu chert crops out about 30 km south of Xigaze and occupies the southern marginal part of the E—W trending Yarlung Zangbo Suture zone. As a result of radiolarian biostratigraphic research along two continuous sections, we identified seven different aged radiolarian assemblages from pelagic and hemipelagic sediments ranging early Middle Jurassic (Aalenian) to Early Cretaceous. The Aalenian radiolarian fauna is the oldest known record so far for the Xialu chert. It is noteworthy to point out that the fauna came from a chert sample which contains no terrigenous elements other than clay minerals. There is a possibility that radiolarian faunas older than Aalenian age will be discovered from the Xialu chert in the future.
基金granted by the National Key Basic Research Program from the Chinese Ministry of Science and Technology(Grant No.2015CB856101)Foundation of Research Center for Tibetan Plateau Geology(21201010000150014-29)the China Geological Survey(No.1212011221066)
文摘Discontinuous ophiolite suites along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet representing remnants of the Neo-Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous.Older ophiolite or ophiolitic mélange(e.g.Triassic)to reveal the initial evolution the Neo-Tethyan ocean within the YZSZ have rarely been documented so far.The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic subbelt,the Zhongba terrane and the southern ophiolitic sub-belt.In this study,we document structural,petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub-belt of the western YZSZ.Two lithos-tectonic sub-units,the southern ophiolitic complex and the northern ophiolitic mélange,are recognized in the Mayoumu massif in terms of structural pattern and petrology.LA-ICP-MS zircon U-Pb dating of gabbro from the ophiolitic mélange yields an age of 243±1 Ma with zirconεHf(t)values of+7.9 to+13.2.Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zirconεHf(t)values ranging from+10.8 to+15.0 and+12.3 to+15.4,respectively.Geochemically,rare earth elements(REEs)patterns show that these mafic intrusions are similar to those of the normal mid-ocean-ridge basalt(N-MORB).Enrichment of fluidmobile elements(e.g.Rb and Ba)and depletion of Nb,and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab.Our data strongly confirms that the Neo-Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least.The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo-Tethyan ocean could be as a continuing process during 120–130 Ma.Emplacement and preservation of older ophiolitic recorder during evolution of the Neo-Tethyan Ocean may be closely related to the occurrence of the Zhongba micro-terrane within the western YZSZ.
基金funded by grants from the NSF China (No. 41672046, 41720104009)the China Geological Survey (DD20160023-01)+1 种基金the Ministry of Science and Technology (201511022)IGCP (International Geoscience Programme) Project 649
文摘The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(>20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.
文摘Meso\| and microstructural characteristics of cataclastic fault rocks developed in the Xialu chert are described. The Xialu chert represents pelagic and deep marine sediments (Middle Jurassic—Lower Cretaceous), and occupies the southern marginal part of the E—W trending Yarlung Zangbo suture zone. The apparent total thickness of siliceous deposits exceeds 1km. The current study is concentrated along the two measured sections, Xialu\|E and Xialu\|W. The Xialu\|E section is composed mainly of red chert associated with reddish purple siliceous mudstone and greenish gray mudstone. The beds steeply dip north or south. The 45m section studied is divided into three units (chert unit, chert\|siliceous mudstone unit, and melange unit from north to south). They are in fault contact with each other. In the southern half of the chert unit and the northernmost of the chert\|siliceous mudstone unit, a 15m thick cohesive cataclasite zone can be defined. The cataclasites are composed of chert fragments in a fine grained matrix, and deformed with jigsaw puzzle structures. The chert\|siliceous mudstone unit is made up of four slabs of chert\|siliceous mudstone sequence. Mesoscopic duplex zones in thickness from 1 to 3m are recognized at slab boundaries. Flat and ramp structure and associated P foliations, R1 shears, Y surfaces are developed. Shear spacing range from 1 to 10cm. The slip senses are dextral strike\|slip movement along these duplex zones. The cataclasite zone is cut by the duplex zone along the southern boundary.
基金funded by grants from the China Geological Survey(DD201190060)the National Science Foundation of China(41672063,41773029,41720104009)supported by the Fund from the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources
文摘The Yarlung Zangbo suture zone(YZSZ)separates Indian plate and its northern passive margin units to the south from Eurasian plate and its active continental margin units of Xigaze forearc basin and Gangdese batholith to the north(Xu et al.,2015;Yang et al.,2015).The western YZSZ in southern Tibet is divided by the Zhongba terrane into the northern belt(NB)and southern belt(SB).Ophiolites in the NB are dismembered as ophiolitic mélanges.Peridotite,cumulated gabbro,ocean plate stratigraphy(OPS)of seamount remnants and pelagichemipelagic sequence as blocks in serpentinite matrix are mainly observed,from west to east,in Dajiweng,Baer,Kazhan,Cuobuzha,Zhalai,Gongzhu.Ophiolites in the SB are absent ophiolitic units of sheeted dikes and MORB-like pillow lavas,occur as much larger peridotite massifs(i.e.,Dongbo,400 km^2;Purang,650 km^2;Xiugugabu,700 km^2;Dangqiong,300 km^2)which are intruded by mafic dike swarms and overlain by volcanic sedimentary OPS(Liu et al.,2018).We propose that the SB mafic–ultramafic rocks and volcanic sedimentary OPS represent fragments of an early Cretaceous continental margin ophiolite whose magmatic evolution was influenced by 140–137 Ma plume magmatism(Liu et al.,2015;Zheng et al.,2019).Relics of Late Paleocene to very Early Eocene deep-marine basin were developed in Saga and Gyirong(Ding,2003;Li et al.,2018).In contract,the NB ophiolitic mélanges report a travel log of an oceanic plate ranging from Middle Triassic to Early Cretaceous.
文摘The Dongbo ophiolite in the western part of the Yarlung-Zangbo suture zone in southern Tibet rests tectonically on the middle-late Triassic and Cretaceous flysch units,and consist mainly of peridotites,mafic dikes,
基金supported by the Project IGCP–649the China Geological Survey(No.DD20160023–01)
文摘The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine (Fos9.7-91.2), orthopyroxene (Enss-92), clinopyroxene (En4-49Wo47-slFS2-4) and spinel [Mg^#=lOOxMg/(Mg+Fe)]=49.1-70.7; Cr^#=(100xCr/(Cr+Al)=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content (0.08-0.62 ppm). Their "spoon-shaped" primitivemantle normalized REE patterns with (La/Sm)N being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites (18.19-50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with 187Os/18Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge (MOR) and were later modified in supra-subduction zone (SSZ) environment.
基金supported by the National Natural Science Foundation of China (Grant Nos.42025201,92155001,41802062,41888101)the Second Tibetan Plateau Scientific Expedition and Research Program (Grant No.2019QZKK0801)。
文摘The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic.However,the evolution of the Neo-Tethys is not well constrained,in particular whether its opening occurred in the Permian or the Triassic and whether a plume was involved with its opening or not.In this study,we present geochronological and geochemical data for mafic igneous rocks in mélanges along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet to constrain the timing and mechanism of opening the Neo-Tethys Ocean.Based on field observations,the YZSZ mélanges can be divided into three segments.The western(west of Zhongba)and eastern(Sangsang-Renbu)segments are composed of ocean plate stratigraphy representing accretionary complexes that formed during subduction of Neo-Tethyan oceanic lithosphere beneath the southern margin of the Asian continent.Mélanges in the central segment(Zhongba-eastern Saga)typically have a siliciclastic matrix,and represent Tethyan Himalayan strata that were structurally mixed with the southern margin of the Asian continent.Based on our and previously published geochemical data,the mafic rocks in the YZSZ mélanges are ocean island basalt(OIB)-like,with ages in the Late Permian-Middle Triassic,the Middle-Late Jurassic,and the Early Cretaceous,respectively.An OIB-like block with an age of ca.253 Ma is identified from the Zhongba mélanges in the western segment,and it is the oldest OIB lithology yet identified in the YZSZ mélanges related to the evolution of the Neo-Tethys Ocean.Geochemical features indicate that this OIB-like block is distinct from typical OIBs and would be formed during continental rifting to incipient seafloor spreading.In the framework of plate divergent-convergent coupling systems and based on literature data for early Middle Triassic seamounts,radiolarian cherts,and normal mid-ocean ridge basalt-like oceanic crust,we conclude that opening of the Yarlung Zangbo Neo-Tethys Ocean would mainly occur at~250–243 Ma in the Early Triassic,not later than the early phase of Middle Triassic.In addition,a mantle plume was not involved in opening the Yarlung Zangbo Neo-Tethys Ocean.On the other hand,we have also identified a suite of ca.160 Ma OIB-like basaltic sills from the Bainang mélanges in the eastern segment,which is the same age as the OIB lithologies previously reported in the Zhongba mélanges.Based on the sill-like occurrence and absence of plume-related rock associations in this region,the Bainang OIB-like rocks might result from Middle-Late Jurassic continental rifting in northern Gondwana.Magmatism related to this tectonic event is preserved in both the YZSZ mélanges and Himalayan strata,but its tectonic significance requires further investigation.Based on this study of the YZSZ mélanges and the previous studies of YZSZ ophiolites,Gangdese belt igneous rocks,and sedimentary rocks,we have reconstructed the entire Wilson Cycle of the Yarlung Zangbo Neo-Tethys Ocean,mainly involving continental rifting and ocean opening,subduction initiation,ultraslow-spreading ridge-trench conversion,subduction re-initiation,and oceanic closure and initial India-Asia collision for the tectonic emplacement of ophiolites.These processes were associated not only with magmatic flare-ups and lulls in the Gangdese belt but also with two stages of ophiolite obduction.Our data therefore provide new insights into the evolution of the Yarlung Zangbo Neo-Tethys Ocean and related Tethyan geodynamics.
基金sponsored by Regional Geology Survey of Lhaze County Sheet(1:250000)(Grant No.19991300009221)the National Natural Science Foundation of China(Grant No.40372061).
文摘Three radiolarian fauna, aged as the Middle-Late Triassic, Late Jurassic-Early Creta- ceous and Cretaceous radiolarian fauna, have been recognised in the radiolarian cherts from the middle sector of the Yarlung Zangbo suture zone, southern Tibet, China. The average contents of SiO2 in the radiolarian cherts of the Middle-Late Triassic and Late Jurassic-Early Cretaceous are 90.24% and 92.58% respectively, with average ratios of Al/(Al+Fe+Mn) as 0.75 and 0.74 respectively, the average ratios of MnO/TiO2 as 0.36 and 1.24, the average ratios of Ce/Ce* as 1.15 and 1.03, and the average ratios of LaN/CeN as 0.85 and 0.93. These geochemical features indicate that both of them are biogenic, deposited in a continental margin basin. The SiO2 content of the Early Cretaceous radiolarian chert is 94.12%, with the ratio of Al/(Al+Fe+Mn) as 0.59, ratio of MnO/TiO2 as 4.30, ratio of Ce/Ce* as 0.60, ratio of LaN/CeN as 1.59, which imply that the chert is biogenic and was deposited in a pelagic basin. The Middle-Late Triassic association of the radiolarian chert and turbidites as well as their geochemical characteristics indicates the existence of a strong rifting marginal basin in the belt of the Yarlung Zangbo River then. The association of radiolarian chert and bedded basalt indicate an initial Tethyan ocean basin in southern Tibet during the Late Jurassic-Early Cretaceous. The early Cretaceous radiolarian chert coexisting with pillow basalt in top of the ophiolite suite represents sediments from the oceanic Tethyan basin.
基金the National Natural Science Foundation of China(Grant Nos.91755209,41972106).
文摘Trench basin,as an important sedimentary repository in oceanic subduction zones,documents faithfully the evolution of paleodrainage and paleogeographic information.Because of the frequent intense deformation during and after deposition,the recognition of trench-basin strata in orogenic belts is quite challenging.Several trench-fill deposits have been identified from the Yarlung Zangbo suture in southern Tibet,which can be classified into two types based on major differences in formation timing and tectonic setting.The first type developed during subduction of the Neotethyan oceanic slab in the Cretaceous(e.g.,the Jiachala,Rongmawa,and Luogangcuo formations),and the second type developed during the initial stage of the India-Asia collision in the Palaeogene(e.g.,the Sangdanlin-Zheya formations).The former was originally deposited on the subducting oceanic crust and then accreted as tectonic slices into the subduction complex;the latter was deposited unconformably on the continental margin of the subducting Indian plate and then involved in the subduction complex during the continental collision.Typical lithologies of trench-basin fills include abyssal chert,siliceous shale,silty to sandy turbidites,debris flows deposits,and slump deposits without carbonate.Detritus feeding these basins were chiefly from the uplifted terrane in the upper plate.This paper summarizes the geological features of trench basins developed in southern Tibet and proposes criteria for recognizing trench-basins in collisional orogens.
基金co-supported by Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(No.GML2019ZD0201)the Second Tibetan Plateau Scientific Expedition and Research Program(No.2019QZKK0801)+3 种基金the National Natural Science Foundation of China(NNSFCProject Nos.42272048,41720104009,42172069,92062215)the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources Fund(No.J1901-28)the China Geological Survey(CGS,Project Nos.DD20230340,DD20221630)。
文摘Understanding the nature of parental melts for pyroxenite veins in supra-subduction zone(SSZ)ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones.The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro,diabase,and basalt.Veins of two pyroxenite varieties cut the southern part of the Zedang massif.These pyroxenite rocks have different geochemical characteristics,where the first variety(type-I)has relatively higher contents of SiO_(2)(51.82–53.08 wt%),MgO(20.08–23.23 wt%),andΣPGE(3.42–13.97 ppb),and lower Al_(2)O_(3)(1.59–2.28 wt%)andΣREE(1.63–2.94 ppm).The second pyroxenite variety(type-II)is characterized by SiO_(2)(45.44–49.61 wt%),Mg O(16.68–19.78 wt%),Al_(2)O_(3)(4.24–8.77 wt%),ΣPGE(14.46–322.06 ppb),andΣREE(5.82–7.44 ppm).Pyroxenite type-I shows N-MORB-like chondritenormalized REE patterns.Zircon U-Pb ages of pyroxenite type-I(194±10 Ma),associated ophiolitic gabbro(135.3±2.0 Ma),and plagiogranite(124.2±2.3 Ma)evidently imply episodic evolution of the Zedang ophiolites.The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction-initiated hydrous melting of metasomatized sub-arc mantle,later overprinted by sub-slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times.The geochemical variations in pyroxenite vein composition,coupled with age differences amongst the other ophiolite units,may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.